This invention relates generally to the field of fluid flow in porous media and more specifically to an apparatus (and a method for the use thereof) for retaining a stack of a variable number of sheet membrane elements, such as filter papers, in the same device while achieving adequate sealing capacity regardless of the number of filter elements in the stack.
A number of applications that require flow through porous media employ a variable number of sheet filter elements, such as filter papers in a stack. A notable example is harvesting biological materials, such as antigens, monoclonal antibodies, virus particles, lymphokines and enzymes by using an affinity binding segregation method. Typically, a filter substrate of paper or other microporous sheet is treated with a substance designed to bond with the substances to be removed from the fluid. The substance with which the filter sheet is treated is referred to as the "ligand." The substance to be removed from the fluid is referred to as the "ligate". The fluid that contains the ligate is brought into contact with the sheet bearing the ligand population. Under suitable conditions, the ligands will bind to the ligates passing therethrough. After a high percentage of the ligand elements of an individual membrane media have bonded to a ligate, the capacity of that particular membrane media may be reached. The membrane sheet still retains its microporous capacity and thus will permit the relatively free flow of fluid therethrough, even after virtual total bonding of the ligands associated with that membrane sheet. It is desirable to be able to stack a number of individual membrane sheets in series to remove a higher percentage of ligate from the solution passing through the membranes. Thus, with a single pass through of solution, a higher yield may be obtained.
During the flow of fluid through the membrane sheet, it is necessary to insure that the fluid flows through the sheet and past the ligands, rather than around a sheet's edges. Therefore, a sealing mechanism must seal off the edges of the membrane sheet or sheets from the fluid flow. In the case of a device capable of holding a variable number of membrane sheets, the sealing mechanism must adequately seal against leakage around the edges of the membrane sheets regardless of the number of sheets used.
After the ligand and ligate associate on the sheet, it is necessary to wash from the sheets any other particles that are not desired. Other particles, which would not be bound to any portion of the membrane sheet, might have become lodged in various microscopic nooks and crannies of the membrane sheet and the vessel. A typical method of washing the undesired particles from the membrane sheets is to flush pure water or another carrier liquid, e.g. buffers, over and through the sheets. The ligand and ligate association is strong enough so that they would not be separated during this mechanical washing. It has been found that it is beneficial to wash the membrane sheet in both the direction of original flow and in a back flow direction, in order to maximize the likelihood that undesired particles will be removed from wherever they may be lodged. Thus, it is necessary that the method of sealing the variable number of membrane sheets adequately seals against leakage when flow is applied in both the forward and backward direction.
Known devices of the prior art with respect to this type of a filtration or particle harvesting method are not capable of sealing a variable number of membrane sheets because the seal structure is sized to accommodate only a fixed number of filter elements, typically a single layer. Further, these known devices cannot be flushed in both directions. All known devices are made for filtration and for that purpose they are adequate. The vessel of this invention provides uniform distribution of fluid over the entire membrane surface area--a different application from simple filtration. It is, however, useful in filtration to achieve uniform surface distribution.